The invention relates to methods for preparing polymer films. In particular, the invention relates to methods of preparing polymer films with improved blocking resistance and reduced coefficient of friction (COF) without the necessity of adding an antiblock or slip agent.
Two side processable films (i.e. two side treated) allow for diverse functionality. For example, the outside of the film may be metallized, printed or laminated and the inside of the film may have cold seal coating and promotional print. Films which are treated on both sides, either with corona or flame treatment, have been used in the past; however, such films have the tendency to become blocked. This is especially a problem for cavitated films or when one of the surfaces is subsequently metallized since blocking leads to metal pick off and loss of functionality. Blocking manifests itself as, among other things, undesirable adhesion between layers of polymer films. Therefore, such films need to be highly loaded with antiblocks or slip agents to reduce blocking and/or reduce COF.
Antiblocking agents (AB) are additives which are incorporated into the skin layers of films to prevent the surfaces of films from sticking to one another or other surfaces. AB usually are finely divided, solid, infusible materials, such as silica, but can be minerals or waxes. They function by forming minute protruding asperities which maintain separating air spaces so as to interfere with adhesion.
Slip agents are modifiers which act as internal lubricants by exuding to the surface of a polymer film during and after processing; whereby, they lubricate the surface to reduce the COF, and thereby, improve slip characteristics.
It has been standard practice for many years to use AB or slip-agents to reduce the COF of the surface of polypropylene films. Thus, blocking of adjacent layers of film on mill rolls is prevented and film can readily be unwound for use or for further processing. In the absence of the slip agent, the layer-to-layer COF is so great that smooth unwinding of the film is virtually impossible. Typical slip or antiblock agents are high molecular weight fatty acid amides such as stearamide or oleamide. It has become standard practice in the art to incorporate about 0.1 to 1%, preferably 0.2. to 0.5% by weight of such compounds into polyolefin films as slip agents.
However, there are many disadvantages with using antiblocking and slip agents in films. Plate-out, for example, is an objectionable coating of these agents which forms on converting or end-use equipment. Additionally, these agents may scratch the opposite metal layer in a metallized film which could damage the barrier properties of the film. Moreover, these agents may interfere or destroy the adhesion of metal coatings to films.
Accordingly, it is one of the purposes of this invention among others, to produce polymer films with improved blocking resistance and surface properties, by providing an economical and relatively uncomplicated method of making polymer films that imparts superior characteristics to the films, without requirement for additives such as antiblock or slip agents.
The present invention is a method providing a layer of an ethylene-propylene (EP) impact copolymer, also known as impact modified copolymer, to multilayer films having a core layer of polypropylene, high density polyethylene (HDPE) or linear low density polyethylene (LLDPE), and a product resulting therefrom. The layer of the EP impact copolymer has from about 3% to about 30% by weight ethylene, and is non-heat sealable. The EP impact copolymer has no more than two peaks in the melting curve within the range from about 110xc2x0 C. to about 165xc2x0 C. The films may be surface treated on one or both skin layers with flame or Corona treatment. The core layer of these films may be cavitated with a cavitating agent such as polybutylene terephthalate (PBT).
The EP impact copolymer layer can be a skin layer or an intermediate layer of a multilayer film. An intermediate layer can be a tie layer. The EP impact copolymer layer can further include ethylene propylene butylene (EPB) terpolymer or polypropylene or ethylene-propylene random copolymer or propylene butylene propylene copolymer or medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE) or high density polyethylene (HDPE) or blends of the aforementioned polymers.
The film can also include a second skin layer which can include high density polyethylene (HDPE) or ethylene propylene butylene (EPB) terpolymer or ethylene-propylene random copolymer or EP impact copolymer or medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE) or propylene butene copolymer or polypropylene homopolymer or ethylene-vinyl acetate copolymer (EVA) or ethylene-methyacrylic acid copolymer (EMA) or ethylene-vinyl alcohol copolymer (EVOH) or blends of the aforementioned polymers. An intermediate layer can be interposed between the core layer and the second skin layer. Such an intermediate layer can include polypropylene and from about 1% to about 15% by weight titanium dioxide. A second intermediate layer can be interposed between the core layer and the EP block copolymer skin layer, and can include polypropylene.
In the embodiment in which the EP impact copolymer layer is an intermediate layer, it is placed adjacent to a skin layer, e.g., interposed between the core layer and a first skin layer. A second intermediate layer can also be interposed between the core layer and a second skin layer, and can include polypropylene or ethylene-propylene impact copolymer. An intermediate layer can be a tie layer. The skin layers can also include high density polyethylene (HDPE) or ethylene propylene butylene (EPB) terpolymer or ethylene-propylene random copolymer or propylene butylene (PB) copolymer or polypropylene homopolymer or medium density polyethylene (MDPE) or linear low density polyethylene (LLDPE) or blends of the aforementioned polymers.
The present invention provides multilayer films with improved anti-blocking and COF characteristics without the necessity of adding an antiblock or slip agent. This gives the ability to produce sealable clear and cavitated films structures, which are processable on both sides, with outstanding surface properties without the use of additives.
These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.